vp9_subpixel_8t_intrin_avx2: fix build w/clang 3.4+

clang reports gcc-4.2.1 in e.g., 3.3, 3.4; add a specific clang version
check for _mm256_broadcastsi128_si256

fixes issue #720

Change-Id: I5c8e3c27fdea05d8a5b050e8cb74894b595f4709